The present invention relates to cellular telephone systems and to modeling cellular telephone systems for optimizing utilization of the available overall radio spectrum. More particularly, the present invention relates to a reliable performance prediction based upon a measurement technique for unobtrusive gathering of data about the performance of the cellular system without interruption of normal operation and for complex analysis of the gathered data.
The service area of a wireless communications system is partitioned into connected service domains known as cells, where radio telephone (cellular) users communicate, via radio links, with the base station serving the cell. The cells can be further partitioned into segments. The base station is coupled to the land line network.
Presently available commercial mobile communication systems typically include a plurality of fixed cells each of which transmits signals to and receives signals from mobile units within its communication area. In AMPS or FDMA systems, each base station is assigned a plurality of channels (each 30 KHz wide) within a frequency spectrum over which it can communicate with mobile units. A mobile unit within range of the base station communicates with the base station using these channels. Typically, the channels used by a base station are separated from one another in some manner (typically skipping 1, 7 or 21 intermediate channels) sufficiently that signals on any channel do not interfere with signals on another channel used by that base station. To accomplish this, an operator typically allots to a base station a group of channels each of which is widely separated from the next. So long as a mobile unit is within the area in which the signal from a base station is strong enough and is communicating with only that base station, there is no interference with the communication. The present invention will also operate with GSM and iDEN systems which do not rely on the same frequency divisions multiple access method.
In a common type of mobile system called Time Division Multiple Access (TDMA), which includes IS-54 and IS-136, GSM and iDEN each frequency channel is further time divided into additional channels within each frequency. Each base station sends and receives in bursts during some number of different intervals or time slots. These time intervals within frequency bands then effectively constitute the individual channels. In order to distinguish the channel divisions within a frequency and to distinguish channels of a common frequency between overlapping cells digital codes are used. For example, IS-136 utilizes Digital Verification Color Codes unique to a channel at a cell, are used. GSM uses Base Station identification codes.
In order to allow mobile units to transmit and receive telephone communications as the units travel over a wide geographic area, each cell is normally physically positioned so that its area of coverage is adjacent to and overlaps the areas of coverage of a number of other cells. When a mobile unit moves from an area covered by one base station to an area covered by another base station, communications with the mobile unit are transferred (handed off) from one base station to another in an area where the coverage from the adjoining cells overlaps. Because of this overlapping coverage, the channels allotted to the individual cells are carefully selected so that adjoining cells do not transmit or receive on the same channels. This separation is typically accomplished by assigning a group of widely separated non-interfering channels to some central cell and then assigning other groups of widely separated non-interfering channels to the cells surrounding that central cell using a pattern which does not reuse the same channels for the cells surrounding the central cell. The pattern of channel assignments continues similarly with the other cells adjoining the first group of cells. Often adjacent or overlapping cells will transmit on the same frequency and both will be received by a mobile wireless unit. However, because of the digital codes identifying each channel, the mobile wireless unit can process the appropriate signal and ignore any additional reception.
It is desirable to provide a process by which interference between cellular telephone system channels operating at the same frequency and/or adjacent frequencies may be accurately predicted over each entire cell of an entire system based upon dynamic information corresponding to in-use performance and for adaptively reallocating channels based upon the in-use interference performance to maximize capacity while minimizing overall interference within the system.
The performance, in terms of service evaluation quality, is the goal of the collection and analysis taught herein. The drive testing measures RF propagation of a cellular system. Performance quality is determined by the analysis of the measured test drive data. Drive testing is used to aid in evaluation of the system, to determine cell placement and channel distribution and to evaluate sector interference. A cellular service area is analyzed and a drive route is established. Determination of a proper and effective drive route is essential for proper system evaluation. The drive route is based on a complex analysis of the system, including its electronic and geographic features. Once a drive route is established, one or more vehicles are driven through the cellular system to collect data. The vehicles are provided with radio receivers for detecting the signal strength from the cell sites and with positioning equipment such as GPS receivers to determine the location corresponding to a received signal. The vehicle are also equipped with data collection equipment, such as computers for collection and correlation of the data.
The measurement system is typically installed in a vehicle. Data is collected as the vehicle travels a predetermined route within the service area. The measurement system can also be used to collect data within a building, although this requires a different mechanism for providing the positioning information. The typical measurement system consists of a scanning receiver, GPS, and laptop computer. The scanning receiver produces signal level measurements for a predetermined list of channels while the GPS provides positioning (latitude/longitude) information. The laptop computer logs the signal level and positioning information during the measurement event.
A scanning receiver tuned to a particular frequency, cannot distinguish between separate signals on that same frequency and therefore does not have the ability to determine if a signal originates from one or more than one cell antenna in an AMPS/FDDMA system. Because of the overlap of signals when all of the cell sites are active, the test vehicles cannot accurately determine a signal level from a given antenna location. A received signal level may be the combination of signals from more than one base station. In order to avoid this problem when drive testing an AMPS system, individual antennas on each sector are keyed-up to a constantly transmitting state on a single particular frequency distinct from all of the other sectors. This is commonly referred to as a keyed-up or beacon signal. By tuning to a particular frequency, a receiver in a test vehicle can accurately determine the base station corresponding to the received signal and can determine the signal level from that single sector.
In key up testing, a unique test channel is established on each sector that propagates into a measurement area. By establishing a unique test channel on a sector, measurements for the given channel can be positively associated to the source sector. This methodology for establishing the unique test channel requires modifications to the system. The test channel is placed in a mode such that it transmits a constant signal, which is referred to as a xe2x80x9ckey-upxe2x80x9d. In addition, the same channel (and sometimes adjacent channels) is turned off (xe2x80x9cblockedxe2x80x9d) on all other sectors that propagate into the measurement area. The process of keying-up and blocking channels requires that many channels be removed from service to perform this measurement collection. Therefore, the measurement collection is limited to periods when there are few mobiles operating in the system, typically 10 pm to 5 am.
If the number of sectors that propagate into a measurement area is greater than the number of key-up channels available, a series of overlapping drives are performed. In this situation, a subset of sectors are assigned key-up channels while the measurements are performed in an area. The area is then re-driven with a different subset of sectors assigned key-up channels. This process is repeated until measurements have been obtained for each sector that propagates into the measurement area.
The goal of collecting measured data is to obtain a complete set of measurements at each location. Complete means that a measurement was achieved for each sector, or it was determined that the sector is below a signal level that can be detected by the scanning receiver.
In TDMA systems IS-54 and IS-136, GSM and iDEN, the signal contains a digital information component identifying the transmitting antenna of a particular signal on a common frequency. It is not necessary to key-up base stations in order to distinguish cell site locations. A drive test can be performed during normal operation of the wireless system. The receiving equipment in the drive test vehicles can determine the transmitting sector based on the Digital Verification Color Code, DVCC, assigned to each sector. The wireless signal for data collection can be acquired by using test equipment designed for TDMA systems, such as the E747A TDMA Drive-Test System from Agilent Technologies and the SeeGull Scanning Receiver from Dynamic Telecommunications. The scanning receivers must be combined with control and positioning equipment as well as data collection and management elements.
The scanning receiver is capable of performing measurements and decoding a xe2x80x9ccolor codexe2x80x9d transmitted on a digital channel. The color code is a digital signature incorporated in information transmitted by the channel. By associating the channel and color code combination detected by the receiver to the combinations known to exist on a sector, the measured signal level can be associated to a particular sector. This assumes that each combination of channel and color code provides a unique identifier, which can be made possible with few modifications to the system.
The ability to decode the color code on a channel is affected by low signal levels and interference. The probability of decoding the color code diminishes due to these factors. Therefore the measurement system may not be capable of decoding a color code at some locations. The resulting data is incomplete due to gaps in the measurements for affected sectors.
In some systems, it is possible to increase the probability of obtaining a color code by attempting rapid decodes on every channel used on a sector. In particular, when the decode is affected by interference, it may be possible to obtain a decode on a channel not receiving interference. There are three likely situations when this could occur:
First, if the interfering signal transitions into momentary fade this may allow an opportunity for reduction of the interference and to decode the signal on the monitored channel.
Second, the combination of channels used in the monitored sector may be different than the channels used on the interfering sector, providing at least one channel not common to the monitored sector and the interfering sector and thus a channel that is not receiving interference.
Third, when channels become inactive when not carrying traffic, it is possible to obtain a non-interference opportunity on at least one channel of a the monitored sector.
These techniques can significantly reduce, but not eliminate, the gaps in the measurement data.
Even with TDMA, DVCC information obtained by drive testing can be incomplete and/or inaccurate. For example, if all of the sectors and channels of a particular site are assigned, the receiving equipment could measure raw RF power, however, it may not be possible for the test equipment to identify a signal at a given location. Momentary systems anomalies can also create drop spots or holes in reception during drive testing, thereby interfering with the collection of signal level measurements on a cellular system utilizing a measurement system. Certain physical conditions, such a bridges and/or tunnels, can create reception anomalies and or increased interference, thereby effecting the normalization of data. Also, the reliability factor of signal measurement, taught herein as an error rate, can cause assignment of varying analysis weight to certain collected data.
The present invention teaches the implementation of data collection weighting and geostatistical analysis techniques in the evaluation of collected drive data. Originally found in the field of mining and petroleum exploration the present invention teaches the application of geostatistical techniques to interpret sparse measurements. The present invention recognizes the applicability of these techniques to the evaluation of drive test data and teaches the method for application of geostatistical analysis to a geospatial region of a cellular system drive test. These robust techniques, when applied as herein taught, are capable of interpolating information for locations where measurements are not available. The process considers the directional correlation of the data to provide an unbiased estimate. In particular, the use of Kriging honors the variable nature of the data in a geospatial relationship.
The applicability of geostatistics to geological exploration can be found for example in U.S. Pat. No. 5,729,451 to Gibbs, et al. Gibbs teaches a data fusion workstation apparatus and method which utilizes algorithms and can be used for applications such as, e.g., hydrogeological modeling, steady-state hydrological flow modeling, transport uncertainty determination, flow/transport fusion, oil reserve management, water supply development and geo-technical engineering projects.
The use of geostatistical techniques, as taught herein, allow the gaps in the measurement data to be interpolated, therefore resulting in a complete set of measurements at each location. In addition, it is possible to interpolate values in areas where measurements where not attempted. For example, it is typically not feasible to obtain measurements for every street in a cellular network. These methods would allow values to be interpolated for streets not measured, and for locations between roads.
Besides actual measurement values, there is information that can be used to provide a more accurate interpolation. The interpolation algorithms can utilize modeled propagation values as secondary information to bolster the estimation. Also, in cases where it may not be possible to associate a measurement to a particular sector (due to inability to decode a color code), the measurement information can still be used to determine an upper bound of the signal level that would be present at a location.
For example, control channels (and key-up channels) transmit a continuous signal. When a measurement is obtained on these channels, it is assured that the signal level received by any sector using that channel in such a mode is below the value measured. This upper bound information can be incorporated in providing a more accurate estimate.
These and other features of the invention will be better understood by reference to the detailed description which follows taken together with the drawings in which like elements are referred to by like designations throughout the several views.